This invention relates generally to monitoring and diagnosing conditions within a processing system. More specifically, the invention is directed to a system and method for gathering diagnostic information based on specially constructed modules or seeds that travel through a flow path of the coal gasification system.
Broadly, gasification is the creation of combustible gas known as synthesis gas and commonly referred to as “syngas” herein, from carbon-containing fuels. Gasification is a well-known industrial process used for converting solid, liquid and gaseous feedstocks using reactants such as air, oxygen, and steam into gases such as hydrogen, carbon monoxide, carbon dioxide, and methane. The resulting gases can be used for generating electrical power, producing heat and steam, or as a feedstock for the production of various chemicals and liquid fuels, or any combination of the above.
In the gasification of a hydrocarbon fuel such as coal or coke, for example, the fuel, in particulated form, is fed into the gasifier reaction chamber together with an oxidizing gas. Reaction of the particulated fuel with the oxidizing gas results in the production of a raw synthesis gas which is carried from the gasifier for further treatment. The events within the reaction chamber produce not only a usable gas, but also a slag having a constituency which depends to a large degree on the fuel being burned and the operating. Because the gasifier for this purpose must be operated at a relatively high temperature and pressure which is well known in the industry, conditions within the combustion chamber must be monitored at all times. Of particular importance, during the initial start-up period when the fuel and oxidant mixture is injected into the reaction chamber, it is essential that the reaction ignition event takes place immediately. Any substantial delay could permit the accumulation of unsafe quantities of fuel and gas to the point where there is the danger of having an uncontrolled explosion within the reaction chamber as well as within other process equipment downstream of the gasifier. It is desirable, therefore, as a safety measure, to monitor the temperature within the gasifier not only during periods of normal operation, but also during the initial startup stage.
Normally, gasifiers are equipped with one or more temperature monitoring devices. One such device is the thermocouple, a plurality of which may be disposed throughout the refractory lined walls of the gasifier reaction chamber. The thermocouples are placed in the gasifier in such a way that they are separated by a thin layer of refractory from the flames in the reaction chamber. This is done to protect the relatively fragile thermocouple junctions from the very aggressive environment inside the reaction chamber. Consequently, the thermocouples do not sense the reaction temperature directly, but instead respond to the heat transmitted through the thin refractory layer from the reaction chamber. It can be appreciated that, as a result of the lag-time inherent in conductive heat transfer, there can be a considerable delay in thermocouple response to critical changes. This is especially true during gasifier startup when reaction initiation results in a rapid temperature rise which must be detected in order to confirm that the reactions have initiated and that unsafe levels of unreacted material are not accumulating within the gasifier and other downstream equipment. In addition, heat transfer lag-times effect thermocouple response to operating condition changes during normal gasifier operation. Thermocouples have also been used as single-point measurement devices within the radiant syngas cooler (RSC).
As an alternative to thermocouples, pyrometers are sometimes used to measure reaction temperature. Physically, the pyrometer is mounted external to the reactor and views the reaction chamber through a gas purged sight tube which normally extends from the pyrometer to the reaction chamber.
A major weakness of the pyrometer temperature monitor arises from the difficulty encountered in keeping the sight tube free of obstructions. The potential for obstruction is great, resulting from the atmosphere within the reaction chamber which is characterized by rapid swirling of particulate carrying gas. Further, a slag which results from ungasifiable material within the fuel, will likewise swirl around the reaction chamber, contacting the walls of the latter. In the course of gravitating towards the lower end of the gasifier, slag normally displays a tendency to cling to the reaction chamber walls. The clinging slag and the swirling particles interfere with the operation of the pyrometer sight tubes which are positioned in the reaction chamber walls. In addition, during the gasifier startup sequence, fuel is introduced into the reactor before oxidant. Depending upon the circumstances and upon the fuel, coal-water slurry for example, there exists an increased tendency for obstruction of the pyrometer sight tubes with unreacted fuel.
These obstructions prevent verification of startup by the pyrometer's response to reactor temperature change. While the problem of obstruction of the pyrometer sight path can in many instances be dealt with by proper adjustment of the sight tube purge gas, there are some difficulties inherent in the use of purge gas itself. If recycled process gas is used, the gas must first be cleaned so that it is entirely free of moisture and particulates, and then compressed for re-injection through the sight tube into the reaction chamber. This may require additional equipment (e.g. oil-free compressor, gas cleaning equipment, etc.) which adds to operations and maintenance expense.
Alternately, if a non-process gas (e.g. an inert gas such as nitrogen) is used as the purge gas, the product from the reaction chamber will be slightly diluted by the pyrometer purge gas. If the gasifier is producing a synthesis gas for a chemical process, the presence of a diluent gas may not be acceptable.
Much simulation has been performed in order to optimize these components, such as the feed injector, the gasifier and the radiant syngas cooler (RSC), and the behavior and thermal profile of the flame proceeding from the injector. However, there has been limited experimental validation of these simulations primary due to the inability of conventional sensors and probes to function or even survive the system's internal atmosphere. In view of the foregoing, the invention overcomes the problems encountered with both thermocouples and optical pyrometers to monitor the actual variables of interest within the gasification flow path.